When a cationic electrocoating or a powder coating is applied to the surface of a metal material, a chemical conversion treatment is generally applied in order to improve the properties such as corrosion resistance and adhesion to a coating film. With respect to a chromate treatment used in the chemical conversion treatment, from the viewpoint of being able to further improve the adhesion to a coating film and the corrosion resistance, in recent years, a harmful effect of chromium has been pointed and the development of a chemical conversion coating agent containing no chromium is required. As such a chemical conversion treatment, a treatment using zincphosphate is widely adopted (cf. Japanese Kokai Publication Hei-10-204649, for instance).
However, since a treating agent based on zinc phosphate has high concentrations of metal ions and acids and is considerably active, it is economically disadvantageous and low in workability in a wastewater treatment. Further, there is a problem of formation and precipitation of salts, being insoluble in water, associated with the metal surface treatment using the treating agent based on zinc phosphate. Such a precipitated substance is generally referred to as sludge, and increase in cost for removal and disposal of such sludge become problems. In addition, since phosphate ions have a possibility of placing a burden on the environment due to eutrophication, it takes efforts for treating wastewater; therefore, it is preferably not used. Further, there is also a problem that in a metal surface treatment using the treating agent based on zinc phosphate, a surface conditioning is required; therefore, a treatment process become long.
As a metal surface treating agent other than such a treating agent based on zinc phosphate or a chemical conversion coating agent of chromate, there is known a metal surface treating agent comprising a zirconium compound (cf. Japanese Kokai Publication Hei-07-310189, for instance). Such a metal surface treating agent comprising a zirconium compound has an excellent property in point of suppressing the generation of the sludge in comparison with the treating agent based on zinc phosphate described above.
However, a chemical conversion coat attained by the metal surface treating agent comprising a zirconium compound is poor in the adhesion to coating films attained by cationic electrocoating in particular, and usually less used as a pretreatment for cationic electrocoating. In such the metal surface treating agent comprising a zirconium compound, efforts to improve the adhesion and the corrosion resistance by using it in conjunction with another component such as phosphate ions are being made. However, when it is used in conjunction with the phosphate ions, a problem of the eutrophication will arise as described above. In addition, there has been no study on using such treatment using a metal surface treating agent as a pretreatment method for various coatings such as cationic electrocoating. Further, there was a problem that when an iron material was treated with such the metal surface treating agent, the adequate adhesion to a coating film and the corrosion resistance after coating could not be attained.
Further, surface treatment of all metals have to be performed by one step of treatment to articles including various metal materials such as iron, zinc and aluminum for bodies and parts of automobiles in some cases. Accordingly there is desired the development of pretreatment method for coating which can apply a chemical conversion treatment without problems even in such a case. Further, there is desired the development of pretreatment method which can apply a chemical conversion treatment without problems as mentioned above, when other coatings using powder coating composition, organic solvent coating composition, and water-borne coating composition besides cationic electrocoating and anionic electrocoating are applied.